1. Field of Invention
This invention relates to a semiconductor laser device to be suitably used as transmission light source in the field of optical telecommunications and optical instrumentation.
2. Prior Art
Data transmission systems having a transmission rate of 10 Gb/sec. or so have been proposed for large capacity trunk lines of the coming generation in the field of optical telecommunications. In this connection, efforts have been paid to exploit the potential of soliton transmission as it serves for optical data transmission without affecting the dispersion characteristics of the optical fibers involved.
Semiconductor laser devices to be used for soliton transmission are required to show a high speed modulation capability for frequencies of 20 GHz and above in order to generate ultrashort pulses of light with a pulse width not greater than 1.times.10.sup.-12 sec. (1 pico-sec.).
Paper 1 listed below discusses some of the results of theoretical researches made on the properties of semiconductor laser devices concerning light wave modulation, using rate equations [1a] and [1b] below for photon density S and carrier density N in an active layer.
Paper 1: IEEE J. Quantum Electronics, Vol. QU-22, p. 833-844, 1086. EQU dS/dt=.GAMMA.G(S,N)-S/.tau..sub.p [ 1a] PA1 .tau..sub.p : the photon lifetime in a laser cavity, PA1 .tau..sub.n : the damping time constant of the carrier, PA1 G(S, N): the laser gain, PA1 J(t): the density of injected carriers including the modulation component and PA1 .GAMMA.: optical confinement factor. PA1 Paper 2: 17th European Conference on Optical Communication, paper Tu. A4.3, Paris, France, September 1991. PA1 N.sub.w : the density of carriers in the quantum well layer, PA1 N.sub.b : the density of carriers in the barrier layer, PA1 V.sub.w : the volume of the quantum well layer, PA1 V.sub.b : the volume of the barrier layer, PA1 J.sub.w (t): the density of carriers directly injected into the quantum well layer, PA1 J.sub.b (t): the density of carriers injected into the barrier layer, PA1 .tau..sub.r : the time required for carriers in the barrier layer to be captured by the quantum well layer (which depends on the time required for carriers to run through the barrier layer or the optical confinement layer) and PA1 .tau..sub.e : the elapsed time (extent) for carriers to be thermally emitted from the quantum well layer, so-called thermionic emission. PA1 Paper 3: 48th Device Research Conference., 5A-31, June, 1990. PA1 Paper 4: IEEE J. Quantum Electron., QE-24, No. 9, pp. 1845-1855, September 1986. PA1 Paper 5: 48th Device Research Conference., Post Deadline Paper 5B-2, June, 1990. PA1 Paper 6: 49th Device Research Conference., Post Deadline Paper 3A-8, June, 1990.
and EQU dN/dt=J(t)-N/.tau..sub.n -G(S,N) [1b]
where,
The modulation response of a semiconductor laser device can be determined by solving rate equations [1a] and [1b] and its highest modulable cutoff frequency is obtained by using equation [2] below. EQU f.sub.m =2.pi.2.sup.1/2 /K [2]
where, K is expressed by equation [3] below. EQU K=4.pi..sup.2 (.tau..sub.p +.epsilon./g.sub.o) [3]
where g.sub.o and .epsilon. respectively represent the linear differential gain and the saturation constant of gain G(S, N) above when it is expressed in terms of threshold carrier density N.sub.t as shown by equation [4] below. EQU G(S,N)-[g.sub.o S/(1+.epsilon.S)](N-N.sub.t) [4]
By studying the above equations, it is evident that the value of K needs to be reduced in order to expand the modulation bandwidth of a semiconductor laser device by increasing frequency fm of the semiconductor laser device and, by turn, it is necessary to reduce p, increase g.sub.o or reduce .epsilon. in order to reduce K.
Known techniques for reducing .tau..sub.p include reducing the length of the cavity and/or the reflectivity of the facet of the semiconductor laser cavity, whereas it is known that g.sub.o can be increased by doping the active layer to turn it into a p-type layer.
However, the values of g.sub.o and .epsilon. are invariable and .tau..sub.p can easily encounter a lower limit because it is correlated with the threshold current density when the semiconductor laser device is prepared by using a single and same semiconductor material such as GaInAsP. Thus, the above identified techniques are not feasible to reduce the value of K for such a semiconductor laser device.
A promising technique is the use of a multiple or single quantum well structure for the active layer in order to multiply the currently available value of g.sub.o by two to three times.
On the other hand, it, is pointed out in Paper 2 below that a quantum well structure can degrade the modulation response of a semiconductor laser device and therefore the time required for carriers to be injected into a quantum well structure is inevitably prolonged because of the specific properties of the quantum well structure.
Now, some of the problems pointed out in Paper 2 above will be summarized below.
The rate equations for a semiconductor laser device having a quantum well structure can be obtained by modifying equations [1a] and [1b] as shown below. EQU dS/dt=.GAMMA.G(S,N.sub.w)-S/.tau..sub.p [ 5a], EQU dN.sub.b /dt=J.sub.b (t)-N.sub.b /n-N.sub.b /.tau..sub.r +N.sub.w /.tau..sub.e (V.sub.w /V.sub.b) [5b]
and EQU dN.sub.w /dt=J.sub.w (t)-N.sub.w /.tau..sub.e +N.sub.b /.tau..sub.r (V.sub.b /V.sub.w)-G(S,N.sub.w) [5c]
where,
A conventional semiconductor laser device having a multiple quantum well structure modifies the density of carriers .DELTA.J.sub.b injected into the barrier layer by reducing the density of carriers injected into the quantum well layer .DELTA.J.sub.w to zero but the modulation response of a semiconductor laser device, or the response of photon density S to .DELTA.J.sub.b, obtained by using such a modulation technique of modulating the carrier density is not desirable because of the reasons as described below by referring to equations [5a], [5b] and [5c].
Firstly, there occurs deterioration in the cutoff frequency as the value of .tau..sub.r increases because the modulation response of the photon density S to the carrier density .DELTA.J.sub.b is directly proportional to (1+jWr), where Wr=.tau..sub.r.sup.-1.
Secondly, the square of the relaxation frequency is inversely proportional to the emission parameter of carriers (.alpha.) as defined by equation [6] below. EQU .alpha.=1+(.tau..sub.r /.tau..sub.e) [6]
Thirdly, an increase in the value of .tau..sub.r and decrease in the value of .tau..sub.e may often be observed, when the optical confinement layer and the quantum well layer are respectively made to be greater than 1,000 .ANG. and smaller than 50 .ANG. in an attempt to achieve a high output level of the device.
When .tau..sub.r increases while .tau..sub.e decreases, the cutoff frequency of a semiconductor laser device is lowered because of the increase in the value of .alpha. attributable to the increased .tau..sub.r and decreased .tau..sub.e values.
Thus, the modulation speed of a conventional quantum well type semiconductor laser device is limited by the slow response of carriers to obstruct any quick modulation of injection current.
This is a problem that surface-emission type semiconductor laser devices commonly experience when they are used for optical interconnection or parallel transmission.
Some of the problems of surface-emission type semiconductor laser devices are pointed out in Papers 3 and 4 below.
Firstly, while the film thickness controllability of a semiconductor multilayer film can be improved depending on temporary film formation in a same grown junction device, there occurs a phenomenon that the resistance of the semiconductor multilayer film becomes high on the p-electrode side. This phenomenon can be observed particularly in InP type multilayer films.
Secondly, a high-speed modulation becomes impossible when a pn junction type current blocking layer is used in the formation of a cavity for a buried structure because of a large parasitic capacity generated there.
Thirdly, if a device is made capable of high-speed modulation, the modulation response of the device can easily be degraded because of a poor thermal dispersion capability of its mesa-type active layer and a large current loss and an increased threshold current level can appear, because the current in the active layer becomes concentrated on the p-electrode side to reduce the current density at the center of the layer and therefore the overlapping area of the injected current and the photoelectric field.